Services should throw exceptions? (Web API) - node.js

Question

I found similar questions but not found any good answer so I am posting it again and with more specific environment.
I am using Node.js + Express to build REST APi for my React Native app.
I am using pattern Router->Controller->Service->Database and I am not sure if I am supposed to throw specific errors from my services.
Example:
I am validating registration request.
Field validations are hapenning inside Controller (I am using ajv JSON schema validator).
Now I need to validate if user/email already exists
Am I supposed to do this from controller by calling for example service.emailExists(data.email) before calling service.createUser(data)??
Or I can let database fall on CREATE duplicate, return false from service.createUser(data) and inform user from controller that Email exists. If I do so, I am not able to inform user if there is Unspecified error inside service, because it will always return as Email exists error, even something else happens.

Answer 1

You can use try...catch, or if...else to handle the possibilities of errors.
This is how it worked for me. Service using express and sequelize
const { Keluhan } = require('../../models');
var getAllKeluhan = () => {
return Keluhan.findAll()
.then(data => {
if (data) {
return {
code: 200,
message: "Data Found!",
result: data
};
} else {
return {
code: 404,
message: "Data not Found!"
}
}
})
.catch(err => {
throw err;
})
}
module.exports = getAllKeluhan;

Answer 2

This kind of typical problem can be solved in different ways. i provide below few.
A general way of handling it to fail in Database layer and there will be a cascading of failure from database layer to service layer and then controller layer. In this case it is assumed that there is no graceful failure and in this case, people just broadcast a generic failure. All the errors and exceptions are reported in a log.
The another approach is to make a failure at the service layer itself if the email id already exists in the cache without connecting to database. It seems to be a good approach as long as there is a good synchronization between database and the cache layer.
Another approach would be a proactive failure where once the user enters the email id and makes a tab out in UI, you can connect to database to validate through a service.
Sometimes, it is also thought that let the user enters everything in the UI screen and let us validate at the end.
As far as design is concerned, I think approach should be given by the functional team who propose the idea. Technically, all the approaches are possible.
The most important thing here is to grab the error or exception in the controller and propagate as a bad request. It means user should be notified whether email id already exists or not.
Finally, you can think of the design like Fail-Safe or Fail-Fast as per your requirements.